Field of the Invention
This disclosure relates, in general, to gas turbine devices, and more particularly, to a hot gas expander inlet casing assembly and method for installing inlet casing components of a hot gas expander.
Description of the Related Art
Energy recovery devices may be utilized in various industries to recover at least a part of energy that would otherwise be wasted. Typically, an energy recovery device is integrated into an industrial process to capture waste energy and extract power therefrom. The recovered energy may be used to drive other equipment in the industrial process, such as an air compressor or a generator. Energy recovery devices increase the overall efficiency of the industrial process and may be utilized in various applications, including blast furnaces in steel mills, cryogenic process devices in air separation plants, and fluid catalytic cracking (FCC) process plants in oil refineries.
One example of an energy recovery device is a hot gas expander commonly utilized in the oil refinery industry. Hot gas expanders capture waste flue gas energy from an FCC process to recover heat energy that would otherwise be lost through one or more pressure reducing valves or throttling valves. Hot gas expanders operate on a turbine engine principle whereby energy is extracted from a flow of hot flue gas. Typically, hot flue gas passes over the nose cone of the expander, past a plurality of stator blades, and engages a plurality of rotor blades. Rotation of the rotor blades relative to the stator blades extracts the kinetic energy of the flue gas and converts it into rotational energy of the rotating shaft which supports the rotor blades. The rotating shaft may power a compressor, a generator, or other equipment.
The rotating shaft and the plurality of rotor blades are disposed inside an expander housing and are enclosed by a shroud assembly fastened to the housing by one or more rings. Typically, the one or more rings are made from a series of arcuate segments that are fastened or welded together. The rings are typically attached to the housing using high-temperature fasteners. The assembly process requires extensive machining to ensure a proper fit of the components. Close tolerances must be maintained in the gap between the rotor blades and the inner surface of the ring immediately adjacent to the rotor blades in order to improve the efficiency of the hot gas expander. Additionally, the arcuate segments must be machined to a high-tolerance in order to assure proper alignment of the bolt holes on the rings with the bolt holes on the housing and/or the shroud assembly. Further machining steps may be required on the housing and/or the shroud assembly to ensure proper seating of the rings.
Within the prior art, U.S. Pat. No. 6,435,820 to Overberg discloses a shroud assembly that includes a plurality of arcuate shrouds held in position by a shroud support which, in turn, is supported by the engine outer case in a conventional manner. The shroud assembly further includes a plurality of retainer plates arranged in an annular array which function to limit aft axial movement of C-clips. The retainer plates may be formed as segmented plates to accommodate thermal expansion. The retainer plate is secured to the shroud support with bolts.
United States Patent Application Publication No. 2007/0253809 to Glynn et al. is directed to a method of assembling a seal assembly within a gas turbine engine. The method includes coupling a stationary stator member to a gas turbine engine including a rotating member, and coupling a primary seal assembly and a secondary seal assembly to the stationary stator member. The primary seal assembly includes a moveable stator member including at least one keyed slot, and at least one biasing member including at least one key. The key is slidably coupled within the keyed slot to facilitate aligning the primary seal assembly and the secondary seal assembly with respect to the gas turbine engine. The seal assembly facilitates sealing between the stationary stator member and the rotating member.
U.S. Pat. No. 7,494,317 to Keller et al. discloses a system for attaching a ring seal to a vane carrier in a turbine engine such that the ring seal may radially expand and contract independently of the vane carrier. The system can also be configured to substantially restrict axial and/or circumferential movement of the ring seal. The ring seal includes a plurality of radial slots circumferentially spaced about the ring seal. A pin can extend substantially through each of the slots and into operative engagement with isolation rings which are connected to the vane carrier.
U.S. Pat. No. 7,686,575 to Chehab et al. is directed to an inner mounting ring for gas turbine flow path components, such as shroud ring segments. The inner ring is mounted to an outer ring on radially slidable mounts that maintain the two rings in coaxial relationship, but allows them to thermally expand at different rates. This arrangement allows matching of the radial expansion rate of the inner ring to that of the turbine blade tips, thus providing reduced clearance between the turbine blade tips and the inner surface of the shroud ring segments under all engine operating conditions.
With reference to FIG. 1, a known hot gas expander 10 includes an inlet casing 20 and a discharge casing 30 attached to a housing 40. Inlet casing 20 receives hot gas from an industrial process while discharge casing 30 exhausts the gas after it has been passed through hot gas expander 10. Housing 40 supports a shaft assembly 50 rotatable about a longitudinal axis 60. Shaft assembly 50 includes a disc 70 disposed between inlet casing 20 and discharge casing 30. Disc 70 includes a plurality of rotor vanes 80 provided on an outer peripheral edge of disc 70. A plurality of stator blades (not shown in FIG. 1) may be provided adjacent to rotor vanes 80 to redirect the hot gases that pass through hot gas expander 10.
Inlet casing 20 may be made of a first material having a first coefficient of thermal expansion, while discharge casing 30 may be made of a second material having a second, different coefficient of thermal expansion. Similarly, disc 70 and rotor vanes 80 may also have a different coefficient of thermal expansion from inlet casing 20 and discharge casing 30. A clearance space is provided between an inner wall of inlet casing 20 and the outermost edges of rotor vanes 80 to account for the dimensional changes of the components during the operating cycle of hot gas expander 10. Differences in coefficients of thermal expansion between these components and their support structures determine the magnitude and variability of the clearance space.
With reference to FIGS. 2-3, another embodiment of the known hot gas expander 10 is shown. In this embodiment, an inner stator shroud 90 is mounted concentrically within an outer rotor shroud 100 inside a retaining groove formed on inlet casing 20. Similarly, rotor shroud 100 is mounted inside a retaining groove formed on inlet casing 20. Stator shroud 90 and rotor shroud 100 are secured to inlet casing 20 by a plurality of mechanical fasteners 110. Stator shroud 90 may have a first half section and a second half section joined at abutting ends. Similarly, rotor shroud 100 may have a first half section and a second half section joined at abutting ends. The plurality of mechanical fasteners 110, such as high-temperature bolts, may be utilized to couple the first half section and the second half section of stator shroud 90 and/or rotor shroud 100.
With continuing reference to FIGS. 2-3, a plurality of individual stator vanes 120 is provided within inlet casing 20. Stator vanes 120 are preferably arranged in a circular arrangement concentric with longitudinal axis 60 of shaft 50 (not shown in FIGS. 2-3). Individual stator vanes 120 are inserted into a retaining groove 130 on inlet casing 20. Stator shroud 90 retains stator vanes 120 in place and prevents their longitudinal movement during operation of hot gas expander 10. Fastening means 110 secure stator shroud 90 to inlet casing 20 at the stator vane 120 inside diameter. Rotor shroud 100 is provided on the outside diameter of rotor vanes 80 and is secured to inlet casing 20 by fasteners 110.
Because components of the hot gas expander are subject to differential thermal expansion during various operating stages of the device, such as powering up to or down from normal operating speed, a large gap between the rotor blades and the inner surface of the ring is often required as a factor of safety for reducing thermal stresses in the energy recovery device. The gap is also a function of material properties for the various components of the hot gas expander. Because the rotor blades are typically manufactured from a different material than the rings, the rotor blades have a different coefficient of thermal expansion compared to the rings. The difference in material properties causes the rotor blades to expand or contract at a different rate and by a different amount compared to the rings during various operating stages of the hot gas expander. This differential expansion may cause a high stress condition in the stator vanes that eventually leads to warping and catalyst entrainment. On the other hand, an increase in the gap between the components leads to a reduction in operating efficiency of the hot gas expander. Such a reduction in operating efficiency is often accompanied by increased operating costs and lower environmental compliance.
Similarly, bolts that secure the rings to the housing and/or the shroud are typically manufactured from a different material than the rings or the housing/shroud assembly. During the various operating stages of the hot gas expander, the bolts expand or contract at a different rate and by a different amount compared to the rings or the housing/shroud assembly. This difference in expansion and contraction causes fluctuations in bolt tension, which ultimately may lead to bolt failure due to material fatigue.